RU2012071C1 - Method for manufacturing fritted pellets of oxides - Google Patents

Abstract

FIELD: nuclear power. SUBSTANCE: initial solution of salts of metal M is treated with hydrogen peroxide in the reactor with "boiling layer" so as to obtain peroxide fall-out in the form of suspension in mother liquors. The fall-out is extracted from the mother liquor, calcined and then reduced to obtain intermediate powder of oxides. The oxide powder is pressed to form pellets which are then fritted. The treatment of strong salt solutions at pH = 2 + 2.5 with hydrogen is conducted by introducing ammonia or ammonium hydroxide into a vertical reactor. The reactor has at least three cylindrical zones of different diameters, one above the other. The central working zone has one end connected to the lower extraction zone of which diameter is less than that of the central zone. The other end of the central zone is connected to the upper overflow zone. Initial reagents are introduced in the working zone while intensively stirring. In addition, salt solutions are introduced into the extraction zone from which suspension of peroxides obtained is discharged. It is possible to recirculate the mother liquor containing fine particles from the overflow zone to the bottom of the extraction zone. Optimal parameters of the process are given in the patent specification. EFFECT: enlarged operating capabilities. 13 cl, 2 dwg, 2 tbl

Description

The invention relates to nuclear engineering, and in particular to technologies for the manufacture of ceramic oxide fuel pellets of the form M _x O _y for nuclear reactors (M is one or more metals from the group of fuel, raw materials and neutron-absorbing elements).

Known methods for the manufacture of fuel pellets, including the steps of precipitating peroxides from solutions of salts (mainly nitrates or sulfates) of elements M, calcining a peroxide precipitate, reducing the precipitate to obtain an intermediate “flowing” (loose) powder of oxides M _x O _y , pressing from said powder oxides M _x O _y , pressing from the indicated powder of tablets and their subsequent frying (sintering) [1,2].

 It is important that these methods eliminate the intermediate steps for conditioning powders suitable for pressing. Such stages include, for example, intermediate sintering of briquettes, their crushing, screening of the desired powder fractions, granulation, etc.

 In the method according to [2], adopted as a prototype, uranium is precipitated in the form of peroxide from a solution of uranyl nitrate with a low concentration of uranium (70-150 g urtane / l) using a weakly concentrated solution of hydrogen peroxide (15-20%). In this case, the deposition process is carried out at a pH in the range from 1 to 2.5 by introducing a mixture of air-hydrate of ammonium oxide. To obtain high density tablets, ammonium nitrate is added to the initial solution of uronil nitrate.

The deposition process is carried out continuously in a cylindrical tank of constant cross section. Reagents are introduced through pipelines entering the tank one above the other in a certain order from the bottom up: a uranium-containing solution, a mixture of air - NH ₃ , a solution of hydrogen peroxide. At the same time, in order to obtain a given (optimal) form of peroxide grains, it is necessary to place the inlet pipes of pipelines at a certain, rather significant distance. The resulting suspension is homogenized by vigorous stirring (three-fold renewal per minute) and completely removed by overflow at the top of the reactor. According to the specified method, uranium peroxide is obtained in the form of spherical grains. The uniformity of deposition is achieved by the configuration of the apparatus.

 The disadvantage of this method is the inability to control and change the particle size distribution of the powder of uranium peroxide due to the fact that small particles are removed during overflow along with the entire suspension. In addition, an additional difficulty is the necessity of introducing ammonium nitrate into the initial solution in excess of ammonium oxide formed upon introduction.

 To solve the problem of regulating the particle size distribution of the powder, it is known from the prior art to use a fluidized bed constant flow reactor. At the same time, grains of a given granulometry are obtained by extracting them from the zone of the reactor in which they are formed. The fines remain in the upper zone of the reactor, where they coarsen before entering the extraction zone, and the concentration of fines in the overflow liquid is minimal.

 The use of such a technology, for example, in the deposition of uranium peroxide from concentrated nitrate solutions is ineffective. There is a production of a significant amount of fines, and even a significant increase in the exposure time of the suspension in the reactor does not allow to control this process. The presence of an excess of fine fraction reduces the fluidity (flowability) of the press powder and reduces the density of the fritted tablets.

 The aim of the invention is to simplify the process while improving the quality of the tablets. An additional goal is to increase the productivity of the installation (fluidized bed reactor).

 These goals are achieved by using a fluidized-bed reactor of a special design, as well as by optimizing the main parameters of the technological process — the flow rate of the mother liquor in different zones of the reactor and the concentration of reagents.

The method of producing fritted (sintered) tablets of M _x O _y oxides, where M represents one or more metals selected from the group of fuel, raw and neutron-absorbing elements U, Pu, Ce, Gd, Hf, Th, includes the following sequential steps: processing the initial solution of salts of the element or elements M using hydrogen peroxide; obtaining a precipitate of peroxides in suspension in mother liquors; the selection of the specified precipitate from the mother liquor; calcining, and then recovering said precipitate to obtain an intermediate powder of M _x O _y oxides; pressing tablets from the specified powder followed by sintering.

 Distinctive features of the claimed method are: the precipitation of hydrogen peroxides from solutions of these salts with a pH maintained in the range of 2-2.5 by introducing ammonia or hydrate of ammonium oxide; carrying out the deposition process in a continuous fluidized bed reactor having at least three cylindrical zones of different diameters mounted on top of each other (interconnected, for example, by transition zones in the form of a truncated cone); the central working zone is connected at one end to the lower extraction zone, the diameter of which is less than the diameter of the central zone, and the other end to the upper overflow zone; while the starting reagents are introduced into the working zone with vigorous stirring, and a solution of salts is additionally introduced into the extraction zone, and the resulting suspension of peroxides is extracted from the same zone.

 In private versions, the diameter of the central working zone is larger than the diameter of the overflow zone, and the overflow zone is equipped with a special drain allowing the mother liquor containing small particles to be recycled from the overflow zone to the base of the extraction zone.

 Particular distinguishing features of the invention are also the following process parameters in the fluidized bed reactor: the velocity of the effluent in the working zone is at least two times lower than the flow velocity in the extraction zone; the speed of the upward flow in the working area is selected in the range of 0.4-1.8 m / h, mainly in the range of 0.7-1.3 m / h; the peroxides are precipitated with a concentrated solution of hydrogen peroxide, the concentration being at least 30%; hydrogen peroxide is introduced in an amount supporting its excess in the overflow zone in the range of 1-10 g / l, mainly in the range of 2-5 g / l; the solids content in the suspension, located in the working area, support in the range of 300-1700 g / l, mainly in the range of 1100-1500 g / l; the concentration of an aqueous solution of uranyl nitrate supplied to the reactor is selected in the range of 50-600 g of uranium / liter, mainly in the range of 250-500 g of uranium / liter.

 In principle, the pH can range from 1.5 to 3.5, however, the above range of 2-2.5 is preferred. With more acidity, an excessive amount of small particles is formed, and with less acidity (i.e., a more alkaline environment), it is difficult to regulate the pH value, which leads to process instability.

 As salts of the elements M, nitrates, sulfates or chlorides can be used, however, the use of nitrates is preferred. Recovery of peroxide sludge is possible in a special furnace in a controlled atmosphere of hydrogen and nitrogen.

The process temperature in the reactor "fluidized bed" is selected in the range of ^about 35-60 C, preferably 35-45 ^{° C} to obtain hydrated uranium peroxide (the residual content of uranium is uranium less than 20 mg / l, preferably 2-15 mg / l) .

 Intensive mixing in the working zone can be carried out, for example, by means of a mixer with vertical blades at a mixing speed of 20-80 rpm, preferably 40-70 rpm.

 The overflow of the upper overflow zone can be connected to the decanter, the decanted fine fraction from which enters the cone of the lower zone, and the light (free from sediment) mother liquors form a processed and / or output effluent.

 Mixing in the upper and lower zones should be moderate so as not to cause harmful homogenization of the layer (whereas in the working zone a significant resumption of contact between the solid and liquid phases should be ensured).

 The initial salt solution is introduced at several, preferably four, points of the working zone in the immediate vicinity of the stirrer blades for instant dispersion of the reagents. Partially, the initial salt solution can be introduced into the extraction base, which increases the amount of fines and, accordingly, reduces the average particle size, i.e., it allows you to adjust the particle size distribution of the resulting powder.

 The extraction of the precipitated product is carried out by means of a sampling pipe made in a cylindrical extraction zone. By adjusting this branch pipe, the flow rate of the product and the solids content in the aqueous suspension, as well as the residence time of the particles in the reactor depending on the ascending speed are established.

 By varying the value of the ascending flow rate, the density of the suspension, the exposure time of the solid substance (solid phase) in the reactor, as well as the fraction of the fine fraction by adjusting the proportion of the initial saline solution entering the base of the extraction zone, it is possible to control the particle size distribution of the powder and the sphericity of particles (optimal size particles is 30-80 microns, and the reactor allows to obtain up to 80% of the particles in the specified interval).

 During the deposition, the introduction of additives that increase the rate of grain enlargement is possible. Such an additive is, for example, isobutyl oleate. To stabilize hydrogen peroxide, additives such as pyrophosphates, benzoates or orthoxyquinoline sulfate are possible.

 After removal from the reactor, the particles are separated from the mother liquors, washed to remove the ammonium salt, which appeared as a result of neutralization by the hydrate of ammonium oxide of the acid formed during the precipitation of peroxide.

The particles are then calcined at a temperature of 350-600 ^about With nitrogen atmosphere, and then restored at a temperature of 400-600 ^about With, preferably at 500-600 ^about With in an atmosphere of nitrogen and hydrogen.

 Obtained according to the described technology, the intermediate oxide powder has a high fluidity. The expiration coefficient established by the MPA standard (Metal Association) ensures the expiration of the powder through the opening of 7.62 mm in 25 seconds, while for powders obtained by known technology, respectively 49-70 s. This circumstance significantly improves the quality of tablets pressed from powder, in particular, it allows to obtain their density in excess of 96% of theoretical density, with uniform porosity and the absence of structural defects.

 In FIG. 1 shows a reactor in which the process of the invention is carried out. In FIG. 1 shows the emptying cone 1, the lower extraction zone 2, the working zone 3-5 formed by the connecting portions of the cone of zone 3 and 5 and the cylindrical part of zone 4, the upper end zone 6-8 formed by the upper cylindrical zone 6, the connecting conical section of zone 7 and overflow zone 8, equipped with overflows 9, channels 10 for supplying a nitrated solution, channels 11 for supplying ammonium hydroxide, channels 12 for supplying hydrogen peroxide, extraction of the suspension 13, paddle mixer 14, counter vanes 15, mounted on the inner wall of the cylinder the working zone, the mixer 16 in the evacuation cone 1, the rotation axis 17, on which the mixers 14 and 16 are mounted, the overflow pipe 18, the overflow pump 19, the evacuation valve 20, a decanter 21, which ensures recycling of the fines fed to the overflow 9 and ensures removal effluent at position 22, a heat exchanger 23, designed to withstand a given temperature of the fluidized bed.

 In FIG. 2 shows a schematic diagram of the reactant inlets when the inlets exit at the level of the end of the oncoming blades in the cylinder of the reactor working zone.

 Example 1. In this example, it is shown that, by carrying out the usual deposition of peroxide in a conventional fluidized bed classifier, it is impossible to obtain a flowing powder with a sufficiently large, uniform and adjustable particle size distribution to obtain fritted tablets with increased density without preliminary processing of intermediate powders of oxides.

 They come from a nitrate solution of uranyl containing 350 g of uranium / l. The deposition of uranium peroxide is carried out in a conventional reactor-classifier with a "fluidized bed" by ascending circulation, providing a bottom-up decreasing particle size classification and solids content in suspension.

 At its base, the reactor contains a cone over which a cylinder with a height of 700 mm and a diameter of 70 mm is installed, with the decantation and overflow zones 100 mm in diameter installed over the last (that is, 2.7 l for the working zone). The reactor is equipped with a low-speed mixer equipped with simple rods, and the overflow enters the decanter, the lower layer of which is recycled to the fluidized bed reactor.

One injection of the initial uranium-containing solution into the cone and two injections into the cylinder are carried out. The total consumption is 120 g of uranium / h; two injections of a solution of H ₂ O ₂ in the cylinder; three injections of NH ₃ into the cylinder with a pH of about 2 in the lower part of the layer and 2.5 in its upper part; one extraction of the resulting suspension located in the lower quarter of the cylinder.

 In this case, there is an increasing production of fines, which is not even regulated by significantly reducing the flow rate and by changing the speed of the upward flow and mixing.

 In this case, the entire uranium-containing solution is introduced into the cone to exclude local oversaturations that cause undesirable fines production.

 The powder obtained during this deposition contains a significant proportion of nonspherical fines, and the particle size distribution is close to 6. Due to poor fluidity, the powder should be subjected to intermediate conditioning treatments in order to be suitable for use with the preparation of fritted tablets.

 In addition, the productivity of 14 g of uranium per hour per liter of core should not be exceeded in order to regulate the production of fines, which is too small for industrial operation.

 Since this powder does not meet the objectives, it is useless to carry out the following steps of the method.

 PRI me R 2. This example illustrates the production of fritted uranium tablets. The initial solution is a uranyl nitrate solution containing 300 g of uranyl / l. Deposition is carried out in a reactor containing a cylindrical working zone with a diameter of 140 mm, a height of 160 mm, connected on both sides by two conical sections with two cylinders with a diameter of 72 mm, the lower extraction cylinder ending with a cone and the upper cylinder connected by a conical section with an overflow cylinder with a diameter 140 mm and a height of 100 mm. The working area has a total volume of 3.6.

 The mixer is formed by one large vertical blade (width 110 mm, height 220 mm), equipped with 4 cutouts on the vertical edges, designed to pass 4 injectors of uranium-containing solution and 4 holes with a diameter of 25 mm, made in a continuous sheet. It is mounted on a shaft located along the axis of the reactor. Two oncoming blades (height 160 mm, width 10 mm) are fixed diametrically opposite on the cylindrical part. It rotates at a speed of 50 rpm. Each of the upper and lower cylindrical zones also contains four flat rods for mixing with a height of 10 mm, mounted on the same shaft.

Reagents are supplied in the working area by 4 injectors of a uranium-containing solution exiting at the level of oncoming blades, two injectors of ammonium oxide hydrate containing 160 g / l exiting at the wall level, an injector of 30% Н ₂ О ₂ located at the base of the lower cylindrical zone .

 The extraction of the suspension of particles is carried out using a sampling pipe that penetrates about 1 cm into the lower extraction part.

The pH is maintained between 2 and 2.5, and the content of H ₂ About ₂ in the overflow is constantly maintained between 3 and 5 g / L. This overflow, which contains undecanted fines, is recycled at the base of the reactor after passing through a decanter in order to recycle the specified fines to enlarge them, classify the particles in the extraction zone and transfer the suspension to a fluidized bed.

 At the first stage, the supply of a uranium-containing solution is gradually brought up to 760 g of uranium / h. After stabilization of the layer, the solids content in the working area is 1200 g / l. The extracted particles are spherical, their average particle size distribution, increasing, reaches 75 microns.

 The content of fines in the overflow, subjected to recirculation, varies from 5 to 38 g / l. This trifle is not found in the extracted suspensions, since the particle size distribution is 2.28–2.45, which means that it is coarsened before extraction.

 At the second stage, two additional feeds of the uranium-containing solution are carried out: one to the cone in the base of the reactor, and the other to the lower cylindrical zone.

 The total supply of uranium is gradually brought up to 1550 g of uranium / h, of which 220 g of uranium / h with two feeds in the lower zone. The solids content in the working area is 1330 g / l.

 The average particle size distribution of the extracted particles gradually reaches 69 μm with a particle size distribution of about 1.6.

 During this production, 8 suspension samples were taken with average particle size distributions from 32 to 69 microns and with particle size distributions from 1.6 to 2.45.

These samples were then filtered and washed, calcined at a temperature of 575 ^{° C} under a nitrogen atmosphere (6 m ³ / h), and then reduced at 600 ^{° C} under a nitrogen atmosphere (2 m ³ / hr) and hydrogen (4 m ³ / h) to obtain a powder of VO ₂ with spherical grains, the characteristics of which are given in table. 1.

 Then these powders were passed through a sieve with cells of 400 μm, without residue on the sieve, then lubricated with a mixture of 0.3% stearate.

Then they were used directly in an automatic tabletting machine to produce crude tablets with a diameter of 9.9 mm and a length of about 15 mm, which were then fritted in a continuous furnace containing a preliminary frying zone at a temperature of 400-700 ^o C, then a frying zone at a temperature of 1750 ^o C in an atmosphere of dry hydrogen (6 m ³ / h).

 Then, the frying tablets were straightened on a diamond wheel.

 The characteristics of the raw and fritted tablets are given in table. 2.

 From the table. Figure 2 shows that all the densities of the fritted tablets exceed 96% of the theoretical density.

 In addition, these tablets have very good heat resistance (density change less than 0.6% of theoretical density) and a very uniform structure (uniform density, no defects).

Claims (13)

1. METHOD FOR PRODUCING FRITTED OXIDE TABLETS M _x O _y , where M represents one or more metals selected from the groups of fuel, raw materials and neutron-absorbing elements U, Pu, Th, Ce, Gd, Hf, including treatment of the initial solution of metal salts M with peroxide hydrogen in a fluidized bed reactor, obtaining a peroxide precipitate in the form of a suspension in mother liquors, isolating the specified precipitate from the mother liquor, calcining, and then recovering the specified precipitate to obtain an intermediate oxide powder, pressing h specified tablets powder followed by frying, characterized in that, in order to simplify the process while improving the quality of tablets, hydrogen peroxide treatment is carried out from concentrated salt solutions at pH 2 - 2.5 by introducing ammonia or ammonium hydroxide in a vertical reactor with a "fluidized bed" containing at least three cylindrical zones of different diameters mounted on top of each other, with the central working zone connected at one end to the lower extraction zone, d the diameter of which is smaller than the diameter of the central zone and the other end with the upper overflow zone, while the initial reagents are introduced into the working zone with vigorous stirring with the formation of peroxide sediment and an additional solution of salts is introduced into the extraction zone from which the resulting suspension of peroxides is extracted.  2. The method according to p. 1, characterized in that the starting reagents are introduced into the working area, the diameter of which is greater than the diameter of the overflow zone.  3. The method according to PP. 1 and 2, characterized in that the mother liquor containing small particles is recycled from the overflow zone to the base of the extraction zone.  4. The method according to PP. 1-3, characterized in that the upward flow rate in the working zone of the fluidized bed reactor is at least two times lower than the flow rate in the extraction zone.  5. The method according to PP. 1 to 4, characterized in that the speed of the upward flow in the working area is 0.4 - 1.8 m / h.  6. The method according to p. 5, characterized in that, in order to optimize the process, the speed of the upward flow is set in the range of 0.7 - 1.3 m / h  7. The method according to PP. 1 to 6, characterized in that the deposition of peroxides is carried out with a concentrated solution of hydrogen peroxide, and the concentration is at least 30%.  8. The method according to PP. 1 to 7, characterized in that the hydrogen peroxide is introduced in an amount supporting its excess in the overflow zone of 1 to 10 g / l.  9. The method according to p. 8, characterized in that, in order to optimize the process, maintain an excess of hydrogen peroxide in the range of 2 to 5 g / L.  10. The method according to PP. 1 to 9, characterized in that the solids content in the suspension located in the working area is 300 - 1700 g / l.  11. The method according to p. 10, characterized in that, in order to optimize the process, the solids content in the suspension is maintained in the range of 1100 - 1500 g / l.  12. The method according to PP. 1 to 11, characterized in that the reactor is fed with an aqueous solution of uranyl nitrate, the concentration of which is 50 - 600 g of uranium / l.  13. The method according to p. 12, characterized in that, in order to optimize the process, a solution of uranyl nitrate with a concentration of 250 to 500 g of uranium / l is supplied.

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